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• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N60/00—Superconducting devices
  • H10N60/20—Permanent superconducting devices
  • H10N60/203—Permanent superconducting devices comprising high-Tc ceramic materials
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N60/00—Superconducting devices
  • H10N60/01—Manufacture or treatment
  • H10N60/0268—Manufacture or treatment of devices comprising copper oxide
  • H10N60/0801—Manufacture or treatment of filaments or composite wires

Definitions

• the present invention relates to superconductors, particularly to the current carrying capabilities of superconductors. It is a proven fact that the critical currents of many superconductors, here polycrystalline superconductors, are limited by grain boundaries formed in them.
• the invention overcomes this limitation of present superconductors by using alterations for improving the current transport properties of the superconductors' grain boundaries. This is done in principle by optimizing the microstructure of the substrate or of a buffer layer system onto which the superconductor is deposited, such that the polycrystalline superconductor contains large grain boundary areas.
• critical current density is the maximum current these conductors can carry as so-called supercurrents in the superconducting state divided by the cross- sectional area A of the superconductor.
• the critical current density is a specific property for a given superconductor, and, for the practical use of a superconductor, one aims to maximize the critical current density.
• Dimos et al. have taught in their publication "Superconducting Transport Properties of Grain Boundaries in YBa 2 Cu 3 O 7- ⁇ Bicrystals", Physical Review B, Vol. 41 , 1990, pp. 4038-4049, that superconducting grains with a small misorientation (below typically 8° to 10°) behave as strongly coupled superconductors whereas larger misorientations (also called large-angle grain bounda- ries) are weakly coupled, showing Josephson junction-like properties.
• the teaching by Dimos et al. is the finding that the detrimental effect of the grain boundaries can be reduced by aligning the superconducting grains with respect to their crystalline main axes.
• wires and tapes of high-7 " c superconductors have been fabricated, the critical currents of which are enhanced by aligning the superconducting grains by a variety of means, such as rolling processes or ion beam assisted techniques.
• these technologies have lead to the fabrication of high-Tc superconductors with current densities of the order of 100 000 A/cm 2 at temperatures of 4.2 K, it remains desirable to fabricate high-T c superconductors with still higher critical current densities or with processes which are less costly and faster than the known ones.
• the effective grain boundary area A ' may be enhanced, e.g., by adjusting the microstructure of the superconductor such that the grains have a large aspect ratio, the long sides of the grains being oriented predominantly parallel to the supercurrent flow.
• Mannhart and Tsuei also revealed a procedure to calculate the critical current as a function of the grain aspect ratio. These calculations show that the critical current strongly increases with the aspect ratio of the grains, being ultimately limited only by the intragrain critical current densities.
• Another method to fabricate conductors made of high-T c superconductors avoids the use of bulk materials and silver (as done in the powder in tube process), but instead enhances the grain boundary critical current densities by aligning the grains by epitaxially depositing superconducting films. It is this technology, called coated conductor technology, about which the invention is primarily concerned. Tapes fabricated by the coated conductor technology are also called conductors of the second generation, as this process has the potential to solve the cost-problem of the first-generation conductors.
• the grain alignment (texturing) required at present is achieved, for example, by depositing the superconductor on a template that has a textured surface (an overview of coated conductors is provided by D.
• Deposition is usually done using standard vapor phase deposition techniques such as sputtering, laser deposition, or thermal evaporation. Recently, non-vacuum techniques like sol-gel methods or dip coating have also been used for this purpose. At present, coated conductors are fabricated predominantly by using three different processes.
• the conductor typically consists of a substrate, for example a metallic tape, a buffer layer system, which usually is based on a series of oxide films, a layer of a high-T c superconductor such as YBa 2 Cu 3 ⁇ 7- ⁇ , and possibly several doping and capping layers.
• a substrate for example a metallic tape
• a buffer layer system which usually is based on a series of oxide films
• a layer of a high-T c superconductor such as YBa 2 Cu 3 ⁇ 7- ⁇
• doping and capping layers possibly several doping and capping layers.
• the grains in the superconductor layer of the coated conductors generally form a two- dimensional network, because the high-T c superconductor is in most cases epitaxially deposited as a polycrystalline film.
• the first technique to produce coated conductors to be described here is known as the rolling assisted biaxially textured technique (RABiTS).
• RABiTS rolling assisted biaxially textured technique
• the Ni-grains become textured along two of their main crystal-axes, so that the grains are aligned in all directions.
• a buffer layer usually composed of CeO 2 and Y-stabilized ZrO 2 , is grown.
• a high-T c material typically YBa 2 Cu 3 ⁇ 7- ⁇ , is deposited as film.
• These epitaxial films reproduce the microstructure of the buffer layer, which in turn has replicated the microstructure of the nickel alloy substrate.
• the thickness of the superconducting films is in the range of a few microns, the entire tapes being 25-50 micrometers thick.
• This process known as the rolling assisted biaxially textured technique (RABiTS) is capable to produce low-angle boundaries (e.g. 5°-8°), consequently, the critical current density is relatively high, reaching values above 10 5 A/cm 2 at 77 K in a field of one Tesla.
• Texturing can also be induced by ion beam assisted deposition (IBAD) or deposition under a glancing angle, which is the so called inclined substrate deposi- tion method (ISD).
• IBAD ion beam assisted deposition
• ISD inclined substrate deposi- tion method
• the buffer layer is textured during growth. This is done in the ISD-process by using a shallow angle between the incoming beam of adatoms and the substrate surface, and in the IBAD technique by irradiating the growing film with additional ions.
• the critical current densities of the superconducting films having again a typical thickness of a few micrometers, exceed 10 6 A/cm 2 at 77 K and zero external magnetic field. A limiting factor for applications of these processes is their low speed, caused by the cumbersome alignment processes.
• the present invention provides the solution to his problem. Summary of the Invention
• a general objective of the present invention is to provide a solution for an increased use of high-T c and other superconducting materials by improving the current transport mechanisms within such materials, in particular for increasing the upper limit of the achievable supercurrent density within such materials.
• a specific objective is to provide an approach by which the current carrying capabilities of superconductors, e.g. superconducting wires or tapes, can be significantly improved in a wide temperature range.
• a further objective of the invention is to provide a technique for a simpler manufacturing process, resulting in cheaper mass production of polycrystalline superconductors with large critical current densities compared with presently used techniques. At present, the fabrication of such superconductors requires cumbersome and costly processes to optimize the grain boundary alignment.
• the novel solution taught by the invention is based on the understanding that the critical current density of coated conductors is a monotonously increasing function of the grain boundary critical current density and of the effective grain boundary area. Therefore, by enhancing the effective grain boundary area, the critical current of such a conductor will considerably exceed the product of the grain boundary critical current density and the geometrical cross-section of the conductor.
• the effective grain boundary area can be readily enhanced.
• such superconductors are the typical superconductors fabricated with the coated conductor technologies.
• a major enhancement of the grain boundary area is achieved by joining two such superconductors with their superconducting sides facing each other, such that a good superconducting contact is established between the two superconducting layers, as sketched in Fig. 3 and Fig. 4.
• the supercurrent can meander along the tapes, in part bypassing the grain boundaries within one layer by shifting into grains of the other layer as illustrated in Fig. 3.
• the supercurrent can readily pass across the grain boundaries associated with the transition from one layer to the others, as these grain boundaries have a very large area.
• a preferable way to fabricate superconductors containing flat grains with large aspect ratios is to use the RABiTS technology.
• the growth is started by using a polycrystalline, textured substrate.
• Such a template layer is routinely produced by conventional metallurgical processes. In case it is desired, but in many cases this will not even be necessary, these grains may also be oriented such that low angle grain boundaries are formed, as is done in the standard RABiTS process.
• Onto this layer a buffer layer and the high-T c superconductor are de- posited as films, such that the shape and the orientation of the grains of the template layer is replicated by the grains of the oxide layer and by the grains of the superconductors.
• Coated conductors containing flat, superconducting grains with large aspect ratios may also be obtained using the IBAD or ISD technologies
• a superconducting layer fabricated by one of these techniques consists of a sheet of flat superconducting grains.
• the critical current density of such a grain- network is given by the critical current density of the so-called limiting path.
• the limiting path is an area crossing the sheet which is composed by those grain boundaries, of which the sum has the smallest critical current of all such areas.
• the effective area of the grain boundaries in the limiting path is approximately given by the product of the width and the thickness of the superconductor. This area is small, as the superconductors are usually thin (about 0.5 ⁇ m to 1.5 ⁇ m).
• the supercurrent can bypass the grain boundaries in a given sheet, by meandering into the other sheet, as shown in Fig. 3.
• the effective grain boundary area available for the meandering is therefore much larger than the grain boundary area limiting the current flow within one isolated sheet.
• the grain boundary area for the current flow within a sheet is approximately the product of the width and the thickness of the grains.
• the grains have a size of the order of 100 ⁇ m*100 ⁇ m*1 ⁇ m (length * width * thickness), so that both areas differ by two orders of magnitude.
• the current can therefore transfer easily from one sheet to the next, even if the critical current density of an intersheet grain boundary happened to be small.
• the relatively small critical current density of high-T c superconductors for c-axis conduction has no limiting effect.
• the supercurrent can readily flow within a 'grain, due to the large value of the intragrain critical current density.
• the current can bypass a grain boundary in the first sheet and then transfer again into the first sheet, in many cases without the need to cross a grain boundary with a low critical current density in the second layer (see Fig. 3).
• the same arguments apply vice versa for a the current flowing in the second sheet. Therefore such a double tape will have a critical current density which will exceed significantly the sum of the critical current densities of the individual sheets, the ultimate limit being given by the intragranular critical current density.
• This area can carry a supercurrent of 250 mA, which, in principle may bypass the grain boundary within the first sheet as it corresponds to a intragrain current density of 5*10 5 A/cm 2 .
• the boundary in the first sheet carries by itself only a supercurrent of 50 mA.
• Another advantage of the invention is that it simplifies the manufacturing proc- ess of superconductors for technical applications by allowing cheaper mass production of polycrystalline high-T c and other superconductors with large criti- cal current densities. This is the case because the large current densities achieved by the invention relax the requirements on the costly and tedious grain alignment.
• Fig. 1 shows a sketch to illustrate that in a polycrystal grain boundaries provided by grains with large aspect ratios (on the right) can have an effective area A' which considerably exceeds the cross-sectional area A of the polycrys- tal.
• the figure has been taken from Mannhart and Tsuei "Limits of the Critical Current Density of Polycrystalline High-Temperature Superconductors Based on the Current Transport Properties of Single Grain Boundaries", Zeitschrift fur Physik B, Vol. 77, 1989, pp 53-59.
• Fig. 2 shows a scanning electron microscopy image of a Ag-sheated Bi- based powder in tube conductor. Interconnected sheet-like BSCCO platelets are clearly visible. The figure has been taken from Y. Yamada et al. "Properties of Ag-Sheated Bi-Pb-Sr-Ca-Cu-O Superconducting tapes Prepared by the Intermediate Pressing Process", Jpn. J. Appl. Phys. Vol. 29, 1990, L 456-458. The micrographs (a), (b), (c) present side-views, as shown in the sketch in the upper left.
• Fig. 3 is an illustration showing how a current can meander between two sheets of polycrystalline superconductors, bypassing the critical current limiting grains boundaries in the sheets.
• Fig. 4 is an illustration showing as an example how a superconductor with a large critical current density can be formed by joining two coated conductors.
• Fig. 5 is an illustration of a cross-section of a superconductor containing two superconducting sheets on two substrates. The substrates are clamped by a mechanical joint, so that the resulting force establishes a superconducting contact between the two superconducting sheets.
• Fig. 6 is a sketch of a superconductor containing six superconducting layers or sheets on four substrates fabricated with a coated conductor process.
• the superconducting sheets are welded together by melting the intermediate layer, which has a lower melting temperature than the two superconducting sheets.
• Fig. 7 is an illustration of a superconductor containing two superconducting sheets deposited two-sided on a flexible substrate. By rolling the sample, the superconducting sheets are brought into contact via an intermediate layer. The superconducting sheets are welded together by melting intermediate layers, which have a lower melting temperature than the two superconducting sheets.
• Example 1 We provide in the following examples for the fabrication of the superconductors according to the invention. As basis for the examples, we use standard coated conductors available at present. Example 1:
• two YBa 2 Cu3 ⁇ 7- ⁇ tapes are used which are fabricated by a standard RABiTS Procedure.
• a Ni-alloy tape e.g., Ni-W with a thickness in the range of 20 ⁇ m to 100 ⁇ m.
• the tape is rolled and heated, so that a surface texture with aligned grains is produced, although the alignment of the grains is not a critical prerequisite for the invention.
• a buffer layer system is deposited.
• Such a buffer layer which itself may consist of various sublayers such as
• CeO 2 /YsZ/Ce ⁇ 2 is deposited on the carrier tape to prevent chemical reactions between the high-T c film and the carrier tape material, or, e.g, the oxidation of the carrier tape during the growth of the superconductor.
• buffer-layers A variety of materials have been found useful as buffer-layers. Beside the YSZ and CeO 2 compounds already mentioned, NiO, LaTiO 3 , MgO or SrTiO 3 have been used successfully, to name a few examples. These buffer-layers are usually grown by standard technologies, such as sputtering, e-beam, or thermal evaporation, pulsed laser deposition or metal organic chemical vapor deposition (MOCVD). But also other non-vacuum techniques may be employed, such as dip-coating, spray-coating, or painting techniques.
• MOCVD metal organic chemical vapor deposition
• Standard materials are, for example, members of the so-called 123-family, comprising the ReBa 2 Cu 3 ⁇ x family, where Re is Y or a rare earth and x is a number of the order of seven.
• additional layers e.g., doping layers may be embedded, as described by P. Grant in "Currents without Bor- ders", Nature Vol. 407, 2000, pp 139-141.
• YBa 2 Cu 3 ⁇ 7- ⁇ a variety of materials, or multilayers may be used, again deposited by a multitude of techniques.
• Standard materials are, for example, members of the so-called 123-family, comprising the ReBa 2 Cu 3 ⁇ x family, where Re is Y or a rare earth and x is a number of the order of seven.
• doping layers may be embedded, as described by P. Grant in "Currents without Bor- ders", Nature Vol. 407, 2000, pp 139-141.
• These layers may be grown by using a variety of film growth techniques, such as such as sputtering, e-beam or thermal evaporation, pulsed laser deposition or MOCVD. But here again, other non-vacuum techniques, like, e.g., dip-coating, spray-coating, or painting processes may be utilized.
• film growth techniques such as such as sputtering, e-beam or thermal evaporation, pulsed laser deposition or MOCVD.
• other non-vacuum techniques like, e.g., dip-coating, spray-coating, or painting processes may be utilized.
• the two tapes fabricated this way are then mounted together with their superconducting layers facing each other.
• the superconductors should not be exposed to any atmosphere causing degradation of the film's surfaces, including humidity. Therefore these process steps have best to be carried out in a nitrogen or oxygen atmosphere.
• a firm contact between the superconducting layers is established by mechanically fixing the tapes, for example by folding and pressing the two substrate tapes as shown in Fig. 5.
• the second example is again based on coated conductor tapes, fabricated as described in example 1 , the difference being that the buffer layers and super- conductors are deposited on both sides of the tapes.
• the high-T c superconductor e.g. YBa 2 Cu 3 O 7 . ⁇
• the intermediate layer is grown, which is also superconducting, but has a lower melting temperature than YBa 2 Cu 3 O 7 . ⁇ .
• such a layer may consist, for example, of a 15 wt% Ag + 0.7 YBa 2 Cu 3 O 7-5
• the superconductor presented as third example is based on a double-sided superconducting tape including a layer with a lower melting temperature than described in example 2. This tape is then rolled up, as sketched in Fig. 7, and welded together like the superconductor in example 2.
• a tape covered with a superconducting layer is folded such that the superconducting layer is folded onto itself and a superconducting contact is established between various areas of one single superconducting layer.
• This folding step can be repeated several times to obtain a stack of superconducting double-layers.

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